On DESI's DR2 exclusion of $\Lambda$CDM

Abstract

The DESI collaboration, combining their Baryon Acoustic Oscillation (BAO) data with cosmic microwave background (CMB) anisotropy and supernovae data, have found significant indication against the $\Lambda$CDM cosmology. This can also be interpreted as the significance of the detection of the $w_a$ parameter that measures variation of the dark energy equation of state. DESI's DR2 article quotes exclusion of $\Lambda$CDM for combinations of BAO and CMB data with each of three different and overlapping supernovae compilations (at 2.8-sigma for Pantheon+, 3.8-sigma for Union3, and 4.2-sigma for DESY5). We show that one can neither choose amongst nor average over these three different significances. We demonstrate how a principled statistical combination yields a combined exclusion significance of 3.1-sigma. Further we argue that, faced with these competing significances, the most secure inference from the DESI DR2 results is the 3.1-sigma level exclusion of $\Lambda$CDM obtained from combining DESI+CMB alone, omitting supernovae.

6 pages with two figures, updated to match version accepted by MNRAS

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• Is Dark Energy Dynamical in the DESI Era? A Critical Review
• Cosmological constraints on Galileon dark energy with broken shift symmetry
• Calibration-independent consistency test of DESI DR2 BAO and SNIa
• Probing departures from $\Lambda$CDM by late-time datasets
• Evidence for evolving dark energy from DESI DR2 BAO and Pantheon${}^{+}$, DES-Dovekie, and Union3
• Model-Independent Dark Energy Measurements from DESI DR2 and Planck 2015 Data
• Comparing measures of the Hubble and BAO tensions in $\Lambda$CDM and possible solutions in $f(Q)$ gravity
• Lowering the Horizon on Dark Energy: A Late-Time Response to Early Solutions for the Hubble Tension
• Breaking Free from the Swampland of Impossible Universes through the DESI Portal
• Diverse reddening distributions in sight lines to type Ia supernovae